Mercury's Surface Resembles Rare Meteorites

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Mercury has a surface unlike any other planet's in the solar
system, instead resembling a rare type of meteorite, researchers
say.

The finding, based on an analysis of data from NASA's Messenger
probe, sheds new light on the formation and history of the
mysterious innermost planet, scientists add.

Mercury, the smallest planet in the solar system, is also one
of the least understood, having received much less attention from
scientific missions than Mars, Jupiter and Saturn. NASA set out
to change that when it launched the Messenger probe a little more
than eight years ago. Messenger became the first spacecraft to
orbit Mercury.

Past research based on Messenger data suggested a vast part of
Mercury is covered with hardened lava, enough to bury the
state of Texas under 4 miles (6.4 kilometers) of once-molten
rock, scientists said. All in all, these mammoth floods of lava
cover 6 percent of the planet's surface, an area equal to nearly
60 percent of the continental United States. They created
Mercury's smooth northern plains between 3.5 billion to 4 billion
years ago. [ Latest
Photos of Mercury by Messenger Probe ]

Lava plains are common in the solar system. For instance, young
Mars spewed lava all across its surface, and it still has the
largest volcano in the solar system: Olympus Mons is about 370
miles (600 km) in diameter, wide enough to cover the entire state
of New Mexico, and 16 miles (25 km) high, three times taller than
Mount Everest.

Now, 205 measurements of Mercury's surface composition, made by
the X-ray spectrometer onboard Messenger, reveal how much
Mercury's surface differs from those of other planets in
the solar system.

"Being the closest planet to the sun does mean its formation
history would be different and more extreme than the other
terrestrial planets, with hotter temperatures and exposure to a
stronger gravitational field," says lead study author Shoshana
Weider, a planetary geologist at the Carnegie Institution of
Washington.

The surface is dominated by minerals high in magnesium and
enriched in sulfur, making it similar to partially melted
versions of an enstatite chondrite, a rare type of meteorite that
formed at high temperatures in low-oxygen conditions in the inner
solar system.

"The similarity between the constituents of these meteorites and
Mercury's surface leads us to believe that either Mercury
formed via the accretion of materials somewhat like the enstatite
chondrites, or that both enstatite chondrites and the Mercury
precursors were built from common ancestors," Weider said.

The researchers also looked at the areas around the northern
volcanic plains. These surrounding locales are more pockmarked by
craters, suggesting they are older, with more time spent getting
scarred by meteor impacts.

The older terrain possesses higher ratios of magnesium to
silicon, sulfur to silicon and calcium to silicon than the
northern plains do, but it also has lower ratios of aluminum to
silicon. These differences suggest the smooth plains came from
magma sources that were chemically different from the source of
the material seen in the older regions.

"The chemical differences between the northern plains and the
surrounding areas, combined with the fact that the northern
plains are younger by about 500 million years, tells us that the
volcanic activity which produced the northern plains involved
melting of different sections of Mercury's mantle, at cooler
temperatures and at a later stage in the planet's history than
the activity that would have produced the older surrounding
terrains," Weider said.

The scientists will detail their findings in an upcoming issue of
the Journal of Geophysical Research-Planets.